WASHINGTON — The White House, confronted by an unexpected crisis on a battlefield it thought it had left behind, scrambled Thursday to reassure Iraq that it would help its beleaguered army fend off militants who have overrun much of the country and now threaten Baghdad.

Recognizing what one official described as an “urgent emergency situation,” President Barack Obama and his aides moved on multiple fronts. A senior official said the president was actively considering U.S. airstrikes against the militant groups. Vice President Joe Biden called Prime Minister Nouri al-Maliki to express U.S. support. And Pentagon officials briefed lawmakers about what one senator later described as a “grave situation.”

In his only public comments on Iraq, Obama said his national security staff was meeting around the clock, but the frenzy of activity has yet to produce a tangible U.S. response, attesting to how swiftly this crisis has erupted and how it has left a stunned White House grasping for answers.

The chaotic situation in Iraq showed no sign of letting up Thursday as emboldened Sunni militants who seized two important Iraqi cities this week moved closed to Baghdad while Kurdish forces poured into the strategic northern city of Kirkuk after it was evacuated by government forces.

Airstrikes were only one of several options being weighed by the president, according to the senior official, who cautioned that the president had made no decision on military action. The airstrikes, the official said, could be delivered by unmanned drones or warplanes.

“I don’t rule out anything,” Obama said, “because we do have a stake in making sure that these jihadists are not getting a permanent foothold in either Iraq or Syria, for that matter.” He said he was watching the events with “a lot of concern.”

For Obama, ordering airstrikes would be a symbolically momentous step, returning the United States to a combat role in Iraq 21/2 years after he pulled out the last U.S. soldier, ending the nation’s involvement in a war that left more than 4,400 Americans dead.

The possibility of coming to Iraq’s rescue raises a host of thorny questions for Obama, who has steadfastly resisted being drawn into sectarian strife in Iraq or neighboring Syria. Republican lawmakers accused him of being caught flat-footed by the crisis and of hastening this outcome by not leaving behind an adequate U.S. force after 2011.

Reports that Iran has sent its paramilitary Quds force to help the struggling Iraqi army battle the militant group, the Islamic State of Iraq and the Levant, raised the possibility that the United States could find itself allied with Iran in shoring up an unpopular Shiite government in Baghdad. The White House said it was aware of the reports but did not confirm them.

Obama insisted he had been monitoring the threat from Sunni militant groups for several months. The United States, he said, had supplied Iraq with military equipment and intelligence.

Until now, though, the White House has rebuffed several requests from al-Maliki for the United States to conduct airstrikes against the staging areas of the militant groups.

In the past two days, Obama acknowledged, it was clear that the United States needed to go further.

“Iraq’s going to need more help,” he said. “It’s going to need more help from us, and it’s going to need more help from the international community.”

The president said the crisis confirmed his decision to reorient U.S. counterterrorism strategy from fighting al-Qaeda in Afghanistan and Pakistan to a more diffuse set of terrorist groups that stretch from the Middle East to North Africa.

On Capitol Hill, however, the images of Baghdad under threat from Islamic militants fanned a political firestorm.

House Speaker John Boehner of Ohio said Obama had been caught “taking a nap.”

Democrats said the strife was the result of former President George W. Bush’s misguided invasion of Iraq in 2003.

Senators on the Armed Services Committee emerging from a classified briefing on Iraq appeared stunned by what they heard from military officials.

A State Department spokeswoman said U.S. contractors working in Baghdad on foreign military sales had been evacuated by their companies. But diplomats and staff members at the embassy in Baghdad and consulates elsewhere in Iraq had not been moved, according to the spokeswoman, Jen Psaki.

While experts said leaving behind a residual force of several thousand U.S. troops would have helped the Iraqi army tactically, some doubt it would have prevented the sectarian forces that are threatening to tear the country into Sunni, Shiite and Kurdish blocs.

“In the long run, I’m not sure it would have made a difference with the forces pushing for the disintegration of Iraq,” said Gen. Amos Yadlin, executive director of Israel’s Institute of National Security Studies.

Andrew Tabler, an expert on Syria at the Washington Institute for Near East Policy, said U.S. airstrikes would “help them deal with the symptoms of the disease, but the disease is rooted in Syria.”

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